Thesis (Ph. D.)--University of Wisconsin--Madison, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

No description available.

The self-consistent field theory (SCFT) has reveived a great success in prediction of the physical properties of a variety of polymeric systems in the recent two decades. However, the traditional SCFT is based on the Gaussian chain model, completely neglecting the chain rigidity effects, which is ascribed to one of the intrinsic properties of polymer chains. This thesis concentrates on the development of SCFT in the framework of the wormlike chain model and studies the influence of the chain rigidity on the chain configuration which directly determines properties of polymer materials in the mesoscale. Firstly, considering Onsager-type, orientational-dependent repulsive interactions, we study a model for the isotropic-nematic interface in liquid-crystals. Through adjusting the ratio of total contour length L to the persistence length lambda, we consider systems consisting of molecules with various degrees of flexibility: from rods to flexible chains. Physical properties such as the surface tension, interfacial width and density- and order-parameter profiles were numerically calculated as functions of the flexibility L/lambda and tilt angle, which is defined as the angle between the interfacial normal and the nematic director. Secondly, We examine the influence of persistency on the phase diagram of AB diblock copolymers and the properties of the phase transition as a function of volume fraction, Flory-Huggins parameter and chain rigidity, covering a broad regime spanning from Gaussian chains to rigid rodlike chains. On one hand, we demonstrate that results from a Gaussian-weight based theory can be recovered in the long-chain limit, and on the other hand, we display that significant revisions to the phase diagram, due to the persistency effects, exist for shorter chains. To achieve this, an efficient numerical scheme is designed for implementing the calculations of the wormlike-chain SCFT in a full six-dimensional space.

The self-consistent field theory (SCFT) has reveived a great success in prediction of the physical properties of a variety of polymeric systems in the recent two decades. However, the traditional SCFT is based on the Gaussian chain model, completely neglecting the chain rigidity effects, which is ascribed to one of the intrinsic properties of polymer chains. This thesis concentrates on the development of SCFT in the framework of the wormlike chain model and studies the influence of the chain rigidity on the chain configuration which directly determines properties of polymer materials in the mesoscale. Firstly, considering Onsager-type, orientational-dependent repulsive interactions, we study a model for the isotropic-nematic interface in liquid-crystals. Through adjusting the ratio of total contour length L to the persistence length lambda, we consider systems consisting of molecules with various degrees of flexibility: from rods to flexible chains. Physical properties such as the surface tension, interfacial width and density- and order-parameter profiles were numerically calculated as functions of the flexibility L/lambda and tilt angle, which is defined as the angle between the interfacial normal and the nematic director. Secondly, We examine the influence of persistency on the phase diagram of AB diblock copolymers and the properties of the phase transition as a function of volume fraction, Flory-Huggins parameter and chain rigidity, covering a broad regime spanning from Gaussian chains to rigid rodlike chains. On one hand, we demonstrate that results from a Gaussian-weight based theory can be recovered in the long-chain limit, and on the other hand, we display that significant revisions to the phase diagram, due to the persistency effects, exist for shorter chains. To achieve this, an efficient numerical scheme is designed for implementing the calculations of the wormlike-chain SCFT in a full six-dimensional space.

Στην εργασία αυτή παρουσιάζουμε μια καινούργια προσέγγιση και διερεύνηση της μεθοδολογίας μέσου αυτό-συνεπούς πεδίου για την μελέτη πολυμερικών συστημάτων κοντά σε επιφάνειες. Η καινούργια μεθοδολογία εισάγει πάνω στο πλέγμα τμηματικές πολυμερικές διαμορφώσεις που αναπτύσσονται σε σχεδόν «συνεχή» χώρο. Τροποποιείται ο παραδοσιακός υπολογισμός της ενέργειας αλληλεπίδρασης ενώ προσδιορίζεται με μεγαλύτερη σαφήνεια η ύπαρξη περιοχής «ισχυρής επιμήκυνσης της αλυσίδας» όπου σύμφωνα και με τις αναλυτικές θεωρίες μέσου αυτό-συνεπούς πεδίου, απουσιάζει το στρώμα μείωσης συγκέντρωσης κοντά στην επιφάνεια. Η ύπαρξη αυτού του στρώματος για μικρότερες πυκνότητες πρόσδεσης αποτελεί ιδιαίτερο χαρακτηριστικό που διακρίνει τις ψήκτρες σε δυο υποπεριοχές (μεσαίας και υψηλής πυκνότητας πρόσδεσης). Η μέθοδος επεκτάθηκε για την μελέτη μακρομορίων σε σχήμα αστεριού. Σε πολύ καλή συμφωνία με το πείραμα παρατηρήσαμε ότι με αύξηση του αριθμού των κλάδων μειώνεται δραστικά η προσρόφηση αυτών των μακρομορίων. Για περιπτώσεις συνύπαρξης στο ίδιο διάλυμα αστεριών με διαφορετικό βαθμό διακλάδωσης παρατηρήθηκε ότι οι λιγότερο διακλαδισμένες αλυσίδες τείνουν να αναπτυχθούν κάτω από τη «ομπρέλα» των πιο διακλαδισμένων. Επίσης μελετήθηκαν πολυμερικά συστήματα μεταβλητής πυκνότητας (χρησιμοποιώντας την καταστατική εξίσωση των Sanchez-Lacombe) για τα οποία υπολογίσουμε τις επιπτώσεις της πίεσης και της θερμοκρασίας στην πυκνότητα στην περίπτωση τήγματος. Διαπιστώσαμε ότι το διάγραμμα τάσης παραμόρφωσης υποδεικνύει μεγαλύτερη αντοχή για γραμμικές αλυσίδες όταν γίνει σύγκριση με αστερόμορφες ίδιου μοριακού βάρους. Αυτό αποδίδεται στην διαφορετική πυκνότητα και μορφολογία των μακρομοριακών διαπλοκών στην περιοχή αλληλοεπικάλυψης προσροφημένων και ελεύθερων αλυσίδων. Στην περίπτωση που η διαδικασία εφελκυσμού είναι αντιστρεπτή τα δείγματα αστερόμορφων μακρομορίων παρουσιάζουν μεγαλύτερη αντοχή λόγω της ικανότητας τους να προσροφόνται ταυτόχρονα και στις δύο επιφάνειες. Η επίδραση της εξωτερικής πίεσης δίνει μη αναμενόμενη συμπεριφορά μόνο σε εξαιρετικά μεγάλες πιέσεις. / -

We develop a self-consistent field theory of polymer dynamics, based on a functional integral approach, which is analogous to the existing equilibrium self-consistent field theory for polymers. We apply a saddle-point approximation to the exact dynamical theory, which generates a set of mean-field equations for the time-dependent density and mean force field. We also develop a method of treating the single-chain dynamics exactly, subject to this mean-field, resulting in a functional Fokker-Planck equation that must be solved along with the mean-field equations in a self-consistent manner. To test the self-consistency, we apply the theory to the simple but non-trivial case of np Brownian particles in one dimension interacting via a short-range repulsion in a harmonic external potential. Results for the non-interacting case agree with the literature. The interacting case demonstrates physically sensible interaction-dependent dynamics, such as an increased broadening of the density field when the repulsion is increased. We also examine the dynamics of a binary system with two distinct particle species. We calculate the center-of-mass trajectories for colliding distributions of species A and B, and observe that when the difference of repulsion strengths between like and unlike species chi is greater than a threshold value (between chi = 0.3 and chi = 0.4), the two species do not mix (indicating the onset of phase segregation).

A common strategy employed to destroy harmful bacteria is to disrupt the bacterial membrane through the action of pore-forming anti-microbial peptides. The manner in which the peptides arrange themselves spatially to form a pore in the membrane, which is important for understanding both the mechanism of pore formation and pore function, is a topic of current debate. We contrast the response of a model membrane bilayer to the presence of solid, cylindrical nanoparticle insertions, when the bilayer is composed of persistent worm-like chains and when it is composed of flexible Gaussian chains. We use self-consistent field theory, with the appropriate single-chain propagator, to describe the amphiphilic star-like triblock copolymers composing the membrane and the solvent. The nanoparticle surfaces are designed to have patches that prefer either the solvent or the tail groups of the copolymers, and the nanoparticles are fixed in space. Using this model with polymers in the lamellar phase, we investigate the question of pore-formation, nanoparticle insertion and hydrophobic mismatch in lipid bilayers and the effect that chain rigidity has on these particular interactions. We find that the main effect of increased chain rigidity is that it increases the free energy scaling and the significance of the energy barriers associated with these pore-forming processes. These results demonstrate the importance of using a more realistic persistent chain when modelling pore formation. / NSERC, CFI, SHARCNET

Techn. Universiẗat, Diss., 2004--Kaiserslautern.

University, Diss., 2005--Frankfurt (Main). / Zsfassung in dt. Sprache.

We develop a coarse-grained model of lipids and proteins in which the lipids are modelled as diblock copolymers and the proteins as rigid cylinders. The generic protein model allows the possibility of amphipathic proteins with intrinsic curvature. Self-consistent field theory (SCFT) is used to determine the morphology of the lipid bilayer in the vicinity of the proteins. In particular, we focus on the case of a long transmembrane protein inserted perpendicular to the bilayer. For this system we use SCFT to determine the mechanical properties of the membrane and the thickness profile as a function of distance from the protein inclusion. The mechanical constants are also used in an elastic theory to predict the thickness profile. Good agreement between the full SCFT and elastic theory is obtained. We also use SCFT to determine systematic trends of the boundary conditions for the thickness profile at the protein interface. Such results could be used as boundary conditions for the description of bilayers using elastic theory. We show that this system undergoes a second order wetting transition as the interaction strength between the protein and membrane is varied. / Thesis / Master of Science (MSc)